Topography and sea effects on MT responses observed in Jeju Island, Korea

Abstract

A magnetotelluric (MT) survey has been performed along four lines in Jeju Island located at South Sea of Korea. Jeju Island is a volcanic island of Quaternary eruption. The purpose of the MT survey was to see if there still remain thermal resources and to see if there exist deeply extended fractures or aquifer system beneath mid-mountain region. The island is of oval shape; 31 km wide and 73 km long, and has famous Mt. Halla of 1,950 m high in its center. Because the island is surrounded by electrically conductive sea and has topographic variation, measured MT data in terms of impedance may be seriously biased by distortion of electric fields. We analyze the topography and sea effects on the measured MT data through three-dimensional (3-D) modeling data which can accommodate topographic variation with edge finite-element approximation to electric field formulation of electromagnetic (EM) responses to subsurface. Because the slope of the island is less than 10 degrees except in the close vicinity of Mt. Halla, the topographic effects on the MT responses appear to be of thickness change of the top layer. The sea effect can be considered as resistive static shift across the short axis of the island.